This invention relates, in general, to subsurface formation testing apparatus, and more particularly to power control methods and apparatus for controlling the performance of non-destructive collection of fluid samples from subsurface earth formations traversed by a borehole.
The sampling of fluids contained in subsurface earth formations provides a method of testing formation zones of possible interest by recovering a sample of any formation fluids present for later analysis at the earth's surface while causing a minimum of damage to the tested formations. Thus, the formation sampler is essentially a point test of the possible producibility of subsurface earth formations. Additionally, a continuous record of the sequence of events during the test is made at the surface. From this record valuable formation pressure and permeability data can be obtained for formation reservoir analysis.
Early formation fluid sampling instruments, such as the one described in U.S. Pat. No. 2,674,313, were not fully successful as a commercial service because they were limited to a single test on each trip into the borehole. Later instruments were suitable for multiple testing; however, the success of these testers depended to some extent on the characteristics of the particular formations to be tested. For example, where earth formations were unconsolidated a different sampling apparatus was required than in the case of consolidated formations.
One major problem which has hampered the reliable testing of subsurface earth formations has been designing a suitable system for controlling the operation of the downhole hydraulic system including numerous solenoid control valves. The typical subsurface formation testing instrument employed in obtaining samples includes a hydraulic power system, such system includes a hydraulic pump which is typically an electrically powered, rotary, positive displacement type hydraulic pump powered by 440 VAC. The hydraulic pump develops hydraulic fluid pressures which by means of solenoid control valves selectively controlled by surface power signals, are used to extend the sample admitting probe and a back-up well engaging pad member and to open fluid sample collection tanks. Examples of such systems can be found in U.S. Pat. Nos. 4,434,653 and 3,780,575. This downhole hydraulic power system represents a large inductive load to the surface power control system. This application of power control signals to the system can result in generating relatively large interference signals. Prior art attempts to eliminate this interference have included using grounded, shielded cable for power wiring and large capacitors for heavy filtering. Such efforts have proven less than totally successful.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing method and apparatus for providing high voltage AC power control signals to the subsurface testing instrument eliminating power surges on the system control and measurements.